Thomas Aquinas, Compendium of Theology

Accordingly, divine providence governs individual beings in keeping with their nature. Since rational creatures, because of the gift of free will, enjoy dominion over their actions in a way impossible to other creatures, a special providence must be exercised over them in two respects. First, with regard to the aids God gives to rational creatures in their activity, secondly, with regard to the recompense allotted for their works. God gives to irrational creatures only those aids by which they are naturally moved to act. But to rational creatures are issued instructions and commands regulating their lives. A precept is not fittingly given except to a being that is master of his actions, although in an analogous sense God is said to give commands to irrational creatures also, as is intimated in Psalm 148:6: "He hath made a decree, and it shall not pass away." But this sort of decree is nothing else than the dispensation of divine providence moving natural things to their proper actions.

The deeds of rational creatures are imputed to them in blame or in praise, because they have dominion over their acts. The actions of men are ascribed to them not only by a man who is placed over them, but also by God. Thus any praiseworthy or blameworthy action that a man performs is imputed to him by the person to whose rule he is subject. Since good actions merit a reward and sin calls for punishment, as was said above, rational creatures are punished for the evil they do and are rewarded for the good they do, according to the measure of justice fixed by divine providence. But there is no place for reward or punishment in dealing with irrational creatures, just as there is none for praise or blame.

Since the last end of rational creatures exceeds the capacity of their nature and since whatever conduces to the end must be proportionate to the end according to the right order of providence, rational creatures are given divine aids that are not merely proportionate to nature but that transcend the capacity of nature. God infuses into man, over and above the natural faculty of reason, the light of grace whereby he is internally perfected for the exercise of virtue, both as regards knowledge, inasmuch as man's mind is elevated by this light to the knowledge of truths surpassing reason, and as regards action and affection, inasmuch as man's affective power is raised by this light above all created things to the love of God, to hope in Him, and to the performance of acts that such love imposes.

These gifts or aids supernaturally given to man are called graces for two reasons. First, because they are given by God gratis. Nothing is discoverable in man that would constitute a right to aids of this sort, for they exceed the capacity of nature. Secondly, because in a very special way man is made gratus, or pleasing to God, by such gifts. Since God's love is the cause of goodness in things and is not called forth by any pre-existing goodness, as our love is, a special intensity of divine love must be discerned in those whom He showers with such extraordinary effects of His goodness. Therefore God is said chiefly and simply to love those whom He endows with these effects of His love by which they are enabled to reach their last end, which is He Himself, the fountainhead of all goodness.